1. Field of the Invention
The present invention relates to an alternatable constant current circuit and in particular, relates to an alternatable constant current circuit which inverts at a high speed current flowing through an inductive load such as in an exciting circuit of an electromagnetic flowmeter and in a slanted magnetic field system of a nuclear magnetic resonance imaging device.
2. Description of Related Art
In conventional art, many alternatable constant current circuits were constituted by combining a DC constant current circuit with a switching circuit for changing-over the polarity.
For example, the alternation of current fed to an inductive load Lx is performed by changing-over the direction of a DC output current with a bridge type switching circuit as shown in FIG. 2 or by changing-over two DC output voltages of positive and negative potentials in a switching power source circuit with a pair of switching circuits as shown in FIG. 3.
In FIG. 2, the output of a DC power source 1 which is obtained by rectifying the output of a commercial AC power source is applied to an inductive load Lx by changing-over the polarity thereof with a bridge type switching circuit 3 constituted by switches S1.about.S4, the load current is detected by a resistor Rs and is compared with a reference voltage Vr, and the resultant error voltage is amplified at an error amplifier EA to drive a transistor Q1 for current control so as to maintain the collector current constant, wherein the current control transistor Q1, the current detecting resistor Rs, the reference voltage Vr and the error amplifier EA inclusively constitute a current regulating circuit 4.
In FIG. 3, the output of a DC power source 1 is converted into a positive voltage VS1 and a negative voltage VS2 with a forward type switching power source circuit 2, these voltages are alternatively changed-over with switches S1 and S2 and are applied to the inductive load Lx, the current flowing through the inductive load Lx is detected by a resistor Rs and the absolute value thereof is compared with a reference voltage Vr after amplified with an absolute value amplifier circuit ABS at a proper degree, the resultant error voltage is again amplified at an error amplifier EA and the amplified output is applied to a PWM circuit PWM to convert the same into a PWM signal, through which a switching transistor Q2 is driven to control the positive voltage VS1 and the negative voltage VS2 in such a manner that the load current is maintained at a predetermined value, wherein the current detecting resistor Rs, the absolute value amplifier circuit ABS, the error amplifier EA, the reference voltage Vr and the PWM circuit inclusively constitute a voltage regulating circuit 5.
With the circuit shown in FIG. 2, in order to rapidly change-over the polarity of a rectangular wave current having a predetermined amplitude flowing through the inductive load Lx, it is necessary to increase the output voltage Vc of the DC power source 1 as large as possible at least during the polarity change-over. However, in the interval when the load current is kept at a constant value after the polarity change-over, the voltage across the inductive load Lx is in principle reduced to zero, therefore when the large value of the output voltage Vc is unchanged, the substantial part thereof is applied to the current control transistor Q1 which increases the power consumption in the current control transistor Q1 to thereby reduce the reliability of the circuit, further the cost for countermeasuring the heat radiation at the current control transistor Q1 increases and the size of the device increases thereby.
For the above reason the output voltage Vc is reduced during the interval when the load current is maintained at a constant value.
In FIG. 3, the current control transistor Q1 as included in the circuit shown in FIG. 2 is eliminated and the output voltages VS1 and VS2 of the switching power source circuit 2 are substantially applied as they are to the inductive load Lx and the resistor Rs so that the power consumption in the current control transistor Q1 as indicated above was eliminated which extremely reduces power consumption in the circuit.
In the circuit shown in FIG. 3, the output voltages VS1 and VS2 are reduced in such a way that the voltage across the inductive load Lx is rendered substantially zero during the time when the rectangular wave current is kept at a perdetermined amplitude, and further during the polarity change-over the voltage across the resistor Rs decreases, such that the output voltages VS1 and VS2 are raised to about their maximum output voltages. However, in the circuit shown in FIG. 3 there are included choke coils CH1 and CH2 and output capacitors C1 and C2 near the inductive load Lx, a parasitic vibration is likely to be induced during the change-over of the output voltages with the switches S1 and S2 and further such vibration is difficult to be suppressed by the feedback loop in the circuit. Namely, in the switching power source circuit the rising rate of the output voltages are controllable, however the lowering rate control thereof is difficult because the output voltage can only be lowered by discharging the current through the inductive load such that once when the output current over-shoots, ringing is induced in the circuit to thereby render it difficult to suppress the parasitic vibration.
FIG. 4 shows a circuit disclosed in JP-A-64-12226 (1989) which is deviced to improve the above problems by combining the advantageous features of the circuits shown in FIG. 2 and FIG. 3.
Namely, the power consumption of the current control transistor Q1 in the circuit of FIG. 2 can be decreased when the collector-emitter voltage Vce thereof is suppressed, therefore in the circuit of FIG. 4 the sum of the collector-emitter voltage Vce and the voltage across the resistor Rs is controlled so as not to exceed a reference voltage Vr2. Further, the parasitic vibration during the current change-over encountered in the circuit of FIG. 3 is suppressed by means of the current control transistor Q1 which serves as a damping resistor.
However, with the circuit shown in FIG. 4 it is difficult to hasten the polarity change-over of the current flowing through the inductive load Lx, namely there remains a problem that a high speed current inversion in the inductive load Lx is difficult, because adjustable voltage range of the switching power source circuit 1 is limited in such a manner that when a lower collector-emitter voltage Vce of the current control transistor Q1 is selected, a higher output voltage Vc can not be obtained.